The present invention is directed to cardiac monitoring and recording apparatus and methods and, more particularly, to an apparatus and method for monitoring the heart activity of a cardiac patient, detecting atrial fibrillation, and recording related cardiac rhythm signals in real time for later interpretation and additional diagnostics.
Cardiac monitoring apparatus for monitoring and/or diagnosing cardiac arrhythmia in a hospital or clinical environment, such as in a cardiac care unit, are generally sophisticated systems. Power and data storage generally do not present significant concerns and excess data can be captured for diagnosis. Such sophisticated systems have not been found to be readily adaptable to ambulatory patients where battery power is required, and only limited digital storage or memory is available. Yet it is desirable to provide a portable system for ambulatory patients that can detect and record an atrial fibrillation event for later interpretation and additional diagnostics by a cardiac professional.
As is well known to those skilled in the art, atrial fibrillation is a potentially life threatening irregular cardiac rhythm. Atrial fibrillation is the irregular and rapid randomized contractions of the atria independently of the ventricles. The atria fire at a very rapid rate, up to 600 beats per minute. Instead of the heart contraction beginning at the sinoatrial node and being conducted to the ventricles, there is a rapid succession of beats at the atria. This localized contraction of the atrial muscle causes the waves to pass round and round in the atrium and leads to no atrial diastole and no atrial heartbeat. Fortunately, only some of these impulses make it through the AV node to depolarize the ventricles, resulting in an irregular ventricular rate. About five million patients are diagnosed with the condition per year, often too late for effective treatment.
The desirability of providing an electronic apparatus and method for automatically detecting atrial fibrillation (“AFIB”) has been recognized for some time. Similarly, it has been recognized as desirable to provide both the detection and the recording of relevant electrocardiogram (“ECG”) signals in a single portable unit that a cardiac outpatient can comfortably wear while the patient follows his daily routine. It is also desirable that such a unit has a low manufacturing cost so that it is more affordable and more patients can benefit from such a unit, yet be accurate so that false detections are avoided. Such a portable unit should be compact, rugged, and lightweight, battery operated with low power consumption, and yet responsive to atrial fibrillation. Similarly, the apparatus and the ECG processing methods it employs should be responsive to atrial fibrillation without being sensitive to extraneous false positives caused by noise or noncardiac muscular activity.
The introduction of large data memory sizes has made available an increased ability to more likely capture data relevant to any cardiac event. However, even large memory units are limited, and some restrictions on data capture must always be invoked. Some prior portable apparatus are programmed to operate continuously for twenty-four hours at which time the data must be uploaded to a central station either for analysis or for storage. Such a daily activity is undesirable where the patient may experience no atrial fibrillation or only a single AFIB event for longer periods of time. Instead, an adaptable system is preferable in which the unit can continuously monitor cardiac activity until an actual AFIB event is detected and captured in memory, the non-AFIB-related data being overwritten continuously. Data can be uploaded or transmitted when AFIB events have been captured.
Although AFIB events have been known to last for extended periods, it has become desirable to record limited data leading up to and following an AFIB event regardless of how long the event continues. For example, a time period of 45 seconds surrounding an AFIB event has been found to provide sufficient data for cardiac analysis. Prior devices, however, may record multiple portions of a particularly lengthy AFIB event indicating that the data represents multiple AFIB events, rather than only parts of a single lengthy one. Such excessive recording is inefficient for a portable unit. Battery power and memory are needlessly expended. Additionally, a cardiac professional's valuable time may be wasted attempting to study various recorded AFIB “events” when in fact the recordings are simply parts of a single event and may provide no further useful information. It has become desirable to provide a method and an apparatus that is better able to discern whether an AFIB event is a continuation of a previous one or is in fact a new event.
Other prior art apparatus contemplate only manually-activated recording when a wearer thinks that he is experiencing unusual heart activity, such as AFIB. The event recorder may be a credit card shaped device that the patient manually places over his heart when he feels that a cardiac “event” is occurring. The event recorder may also be mounted to the belt of the patient or be suspended about the neck of the patient with a lanyard and may take the form of a small pager-shaped device that is continually connected to the patient's chest skin via electrodes and a lead set.
Typically such event recorders “record” a patient's ECG for several minutes when a cardiac “event” is occurring in the patient. When the patient feels that an event is occurring, he activates the recording mode by pushing a button and the device begins recording. The recorder then records the ECG waveforms in digital form for later transmission over a telephone line or other digital communications means, such as through an RS-232 connection. Whenever patient transmission is required, the patient holds the speaker of the recorder to a telephone device mouthpiece (or connects another type of digital communications device) and uploads the recorded data to the computer of the health care facility providing cardiac monitoring services to the patient. The device includes a small speaker that converts the stored digital ECG data into audio form for transmission over the telephone line to a telephone receiver at his physician's office or other central medical facility. The audio data transmission is then converted back into digital and/or graphical form at the physician's office and the data examined.
In the past, such event recorders have been provided but simply record all ECG data occurring after a patient has activated the recorder. As discussed above, memories have limited capacities and in the case where a patient is under care only for the specific ailment of atrial fibrillation, only data leading up to, and following, an atrial fibrillation event may be of interest. Additionally, it has been demonstrated that persons are likely to only recognize about 20% of clinically significant events during waking hours, and even fewer while asleep. Thus, an automated AFIB detection system that accurately detects an AFIB event and automatically stores relevant data to that event is desirable.
Once a patient has been identified for AFIB monitoring, it is desirable to filter out other irregular cardiac rhythms, even though they may indicate the presence of heart disease. In particular, premature ventricular contraction or “PVC” (extra systole) is a cardiac contraction occurring before the normal one. PVC's should be ignored as they are not an AFIB event and it is not desirable to record ECG data surrounding such contractions. As another example, regularly recurring cardiac rhythms should be ignored as they also are not AFIB events. Bigeminy is a cardiac arrhythmia in which every other beat is a ventricular ectopic or premature ventricular contraction. Two beats follow each other in rapid succession, each group of two being separated by a longer interval. The initial beat is regarded as the normal one and is usually of greater intensity than the second one. Trigeminy is a series of three beats in rapid succession. Both bigeminy and trigeminy are irregular cardiac rhythms but they are not considered to be AFIB. It would be desirable to ignore them in monitoring a patient's cardiac rhythms for the existence of AFIB. Further, bigeminy and trigeminy have been found to regularly recur in a patient where AFIB is irregular.
It has been known to measure the R—R interval between sequential QRS complexes to detect cardiac arrhythmia. Running averages of the R—R interval have been used as well as other data processing techniques to identify various arrhythmia. However, these techniques have typically been used to identify a large set of arrhythmia and have not been focused on detecting and recording AFIB events. Such prior processing techniques do not clearly identify to a cardiac professional that an AFIB event has occurred and typically, extensive irrelevant ECG data must be examined to locate the actual AFIB event therein, if one even occurred. PVC's as well as regularly recurring arrhythmia (bigeminy and trigeminy) may trigger the recorder in the prior devices thereby using up valuable memory, battery power, and requiring unnecessary uploads of data when an AFIB event has not occurred. Thus, it would be desirable to more accurately recognize an AFIB event and record only ECG data related to that event.
Hence, a need has been recognized by those skilled in the art for an apparatus and a method for more accurate detection of AFIB, for fewer detections of false AFIB events and other non-AFIB events, and for uploading data from the portable unit to the central unit only when the portable unit has recorded AFIB data. Additional needs have been recognized for a light weight, portable unit that can be worn by a cardiac outpatient without substantially interfering with his daily routine, that is automatically responsive to AFIB events, that records only clinically significant AFIB events while minimizing false spurious recording, that significantly improves the power efficiency of the unit so as to permit batteries to last longer, and that has an enhanced signal-to-noise ratio for reduced sensitivity to extraneous or false positives. The present invention fulfills these needs and others.